Optical disks such as CD (compact disk) and DVD (digital versatile disk) and optical card memories have been used as optical information-recording media.
For further improvement of the volume of recorded information, an information-recording medium shown in FIG. 20 having multiple recording layers 101 three-dimensionally laminated is described in Nonpatent Document 1.
The information-recording medium 121 has a transparent glass substrate 104 and recording layers 101a to 101d of a urethane-urea copolymer material of a photon-mode material and intermediate layers 102a to 102c of a film of PVA (polyvinylalcohol) and PMMA (polymethyl methacrylate) laminated alternately on the transparent substrate 104.
During recording, a high-peak-power recording light 122a having a wavelength of 0.790 μm that is emitted from a Ti-sapphire-laser recording-light source 120a is transmitted through a beam splitter 118a, expanded in beam diameter by a beam expander 123, transmitted through another beam splitter 118b, and converged by an object lens 106 into a desirable recording layer 101c of a multilayered three-dimensionally recordable/reproducible information-recording medium 121 (convergent light 107). When such a convergent light 107 is focused, the region where the power density of light is higher (convergent point and its surrounding region) absorbs the light as if the wavelength became half by a nonlinear phenomenon such as a two-photon absorption process, forming a recording pit 105. Accordingly even when the number of recording layers is increased, the other recording layers are almost transparent to the recording light, and the two-photon absorption occurs only in a particular recording position, and thus, it is possible to avoid decay of the recording light and perform recording sufficiently in the lower recording layer.
On the other hand, during reproduction, a low-peak-power reproducing light 122b at a wavelength of 0.6328 μm emitted from a He—Ne laser of reproducing-light source 120b is similarly converged by an object lens 106 into a recording pit 105 in the desirable recording layer 101c (convergent light 107). The reflected light is bent by the beam splitter 118b into the Y-axis direction, converged by a detection lens 111, transmitted through a pinhole 114 placed at the convergent point of the detection lens 111, and detected by a photodetector 119, allowing a signal to be reproduced.
Although not discussed in Nonpatent Document 1, in conventional optical disk recording/reproducing devices, focus servo control is performed to make the recording or reproducing light from its light source focus though an object lens on a disk accurately during recording or reproduction. It is thus preferable to perform the focus servo control also when a recording pit 105 formed by nonlinear recording such as two-photon absorption recording as in Nonpatent Document 1 is recorded or reproduced. Accordingly, if a certain reflected light is obtained from the recording layer 101 when the recording light 122a or the reproducing light 122b is irradiated on the information-recording medium 121, it is possible to focus the recording light accurately in a desirable recording layer 101c by using the reflected light as the light for focus servo control.
However, studies by the inventors showed that there were the following problems occurring when such focus servo control is performed in a three-dimensionally recordable information-recording medium 121 such as that described in Nonpatent Document 1.
Namely during recording or during reproduction of the information-recording medium 121, reflected lights for focus servo control both at the recording-light wavelength and the reproducing-light wavelength are needed at a certain intensity in each recording layer 101 for converging the recording or reproducing light in a desirable recording layer 101c accurately.
However, the information-recording medium 121 has multiple laminated recording layers 101 for forming recording pits three-dimensionally. Thus when the number of the recording layers 101 is large, the recording and reproducing lights are reflected and adsorbed not only in a desirable recording layer 101c but also in other recording layers 101. Reflection and absorption in each recording layer 101 lead to decrease of the intensity of recording and reproducing lights in the bottom layer most separated from the object lens (101d in FIG. 20). Accordingly, use of the recording or reproducing light for obtaining the reflected light for focus servo control results in decrease of transmission of the recording or reproducing light, causing a problem that favorable recording pits are not formed in lower recording layers especially during recording demanding high light intensity for example in two-photon absorption process. Specifically, when nonlinear recording such as two-photon absorption recording, multiple-photon absorption recording or plasma absorption recording is used, the recording sensitivity decreases more drastically with decrease in light intensity especially during recording than that in common one-photon absorption recording. For example in the case of two-photon absorption recording, the large decrease in recording sensitivity occurs, because the recording sensitivity is proportional to the square of the light intensity (with the nth power in n-photon absorption recording). For example in normal one-photon absorption recording, the recording sensitivity decreases to 0.8 times when the intensity of recording light becomes 0.8 times, but it decreases to the square of the value, 0.64 times, in two-photon absorption recording. Thus in a conventional information-recording medium as described in Nonpatent Document 1, it was difficult to assure a reflected light for focus servo control and also a sufficient light intensity in the lower recording layer during recording. Although it may be possible to adjust the power of the recording-light source according to the position of the recording layer in such a case, because the nonlinear recording demand a high-peak-power semiconductor laser, for example at several 100 mW to 1 W or more, as the recording-light source, it is unrealistic to adjust the power of the light source toward the upward direction. Thus in forming recording pits three-dimensionally in a recording part, it is difficult to make the power of recording-light source for that purpose higher than that for forming recording pits in upper recording layers. For that reason, there is a need for an information-recording medium that demands no power adjustment during recording or allows favorable recording with smaller power adjustment (for example, adjustment amount of not more than 30%).
Nonpatent Document 1: Yoshimasa Kawata et al., “Three-dimensional optical memory by using an organic multilayered medium”, Optics Japan 2000, Preprint pp. 95-96 (2000)